This invention relates to a battery pack provided with a voltage detection circuit to detect battery voltage, and in particular to a battery pack that detects the voltage of a plurality of batteries with a plurality of voltage detection circuits.
In a battery pack housing a plurality of batteries, detecting the voltage of each battery and controlling charging and discharging can reduce battery degradation. This is because over-charging and over-discharging can be prevented for all batteries by suspending battery pack discharging if any battery becomes fully discharged and its voltage drops to a minimum voltage, and by suspending battery pack charging if any battery becomes fully charged and its voltage rises to a maximum voltage. Battery voltage for all batteries of the battery pack is detected by either a single voltage detection circuit or a plurality of voltage detection circuits. A battery pack, which detects battery voltage of all batteries with a single voltage detection circuit, detects the voltage of many batteries with that single voltage detection circuit. Therefore, not only does the voltage detection circuitry become complex, but it is necessary to use a specially designed voltage detection circuit. A battery pack, which detects battery voltage with a plurality of voltage detection circuits, can detect voltage for all batteries using standard voltage detection circuits designed to detect the voltage of specific channels.
A battery pack has been developed that detects the voltage of a plurality of batteries with a plurality of voltage detection circuits. A block diagram of this battery pack is shown in FIG. 1. This battery pack has the drawback that the voltage detection circuits 32 draw unbalanced discharge currents from the batteries 31. This is because, although equal discharge currents flow to ground from all batteries 31 via the output terminal of the upper most voltage detection circuit 32 as shown by arrow A of FIG. 1, discharge currents from only some of the batteries 31 flow from the middle and lower voltage detection circuits 32 as shown by arrows B and C. Current that flows from the output of a voltage detection circuit is a small current. However, since that current flows continuously, it results in unbalanced battery discharge.
A battery pack, which balances currents by connecting balance resistors in parallel with the batteries, has been developed to prevent this drawback (Japanese Patent Application Disclosure HEI 11-233154 (1999))
As shown in the circuit diagram of FIG. 2, the battery pack cited in the prior disclosure has expended current adjust resistors 33 connected in parallel with specific batteries 31. The expended current adjust resistors 33 are connected to balance current flow through the paths shown by arrows B and C of FIG. 1. Specifically referring to FIG. 1, the expended current adjust resistors 33 are connected to supplement batteries V1-V3 with the same current that flows in the path of arrow C, and likewise to supplement batteries V1-V6 with the same current that flows in the path of arrow B. Consequently, the expended current adjust resistors 33 balance the unbalanced currents drawn from the batteries 31 by the voltage detection circuits 32.
However, if the output current of a voltage detection circuit changes, current flow through the batteries in this type of battery pack becomes unbalanced. The function of a voltage detection circuit is to compare battery voltage with a set voltage, and depending on whether battery terminal voltage is above or below the set voltage, switch the output to a HIGH or LOW voltage. A battery pack that switches a battery output switch on or off depending on a HIGH or LOW signal issued from a voltage detection circuit cannot always discharge all batteries in a balanced fashion. For example, if expended current adjust resistors are set to balance battery currents when voltage detection circuit output is in a HIGH state, battery currents cannot be balanced when voltage detection circuit output switches to the LOW state.
The present invention was developed with the object of further resolving the drawbacks described above. Thus it is a primary object of the present invention to provide a battery pack that can prevent unbalanced battery discharge, and can discharge all batteries in a balanced fashion at all times while detecting the voltage of a plurality of batteries via a plurality of voltage detection circuits.